1. Field of the Invention
The present invention relates to corrosion-resistant silicon nitride ceramics suitable for members used at high temperature, in particular, to corrosion-resistant silicon nitride ceramics suitable for use as heat engine pants such as gas turbine engine parts.
2. Description of Related Art
Since silicon nitride traditionally known as engineering ceramics is excellent in heat resistance, thermal shock resistance, wear resistance, and oxidation resistance, the application of silicon nitride to heat engine parts such as gas turbines and turbo rotors is particularly proceeding.
For such silicon nitride ceramics, the characteristics of high density and high intensity can be obtained generally by adding sintering additives. For example, silicon nitride ceramics can be obtained by adding sintering additives such as Y2O3, Al2O3, and MgO to silicon nitride powder, followed by firing. This silicon nitride ceramics is usable in a high temperature range environment of not less than 1000° C. without cooling, in which it is impossible to use any metal material. This allows a small gas turbine to achieve a thermal efficiency as high as 40% and higher, which is not attainable with a traditional metal material.
Meanwhile, when silicon nitride ceramics is used as a heat engine member such as a gas turbine member, strength properties is not the only characteristic required in a high temperature environment. That is, it is required to have a high resistance to corrosion due to high temperature airflow, and also required to be excellent in wear resistance and shock resistance to collision with fine particles.
Silicon nitride ceramics, however, suffers from the problem that it reacts with high temperature moisture contained in the gas turbine combustion gas and is corroded and consumed, resulting in a remarkable short lifetime. The recession of ceramics due to high temperature combustion gas containing moisture is particularly pronounced in components such as combustor liners, transition ducts, and nozzles used in gas turbines.
For this reason, in addition to consideration of sintering additive and grain boundary phase, and consideration of firing condition, attempts to improve corrosion resistance by forming a coating layer on a substrate surface are being pursued. For example, Japanese Patent Application Publication Laid-Open No. 5-238859 discloses an attempt to improve peeling resistance as well as corrosion resistance by forming a coating layer composed of an under layer, an intermediate layer, and a surface layer, which are of oxide selected from zircon, zirconia, alumina, mullite, and yttria, on the surface of a silicon nitride ceramic substrate in the order in which thermal expansion coefficient increases from the substrate.
With the members described in this publication No. 5-238859, the residual stress due to the thermal expansion coefficient difference between the substrate and the surface layer can be relaxed by disposing the respective layers such that the thermal expansion coefficient increases sequentially. However, in the environment in which start and stop are repeated to cause a heat cycle involving a large temperature change as in a gas turbine, it is impossible to prevent extension (propagation) of a crack occurred in the surface layer. As a result, the crack can reach the substrate, which causes the problem of peeling in the coating layer such as the surface layer.